A conductive coating film has a broad range of use such as shielding an electromagnetic wave generated from a cathode-lay tube, a plasma display panel and the like, an infrared ray shielding film of a constructive material or an automobile, an anti-static coating film of an electronic equipment or a cellular phone, an anti-fogging hot coil on glass, wiring for a circuit board, coating for imparting conductivity to a resin, and a circuit itself and the like. As methods for forming such conductive coating films, there are conventionally known methods such as a vacuum metal deposition method, a chemical deposition method, an ion sputtering method and a method in which metal particles are dispersed in a dispersing medium and the resulting metal colloidal solution is applied, heated and sintered. However, these methods have problems such that the complicate operation is needed, mass producibility is inferier, and heating at high temperature is required, etc.
Meanwhile, there is known another method forming wiring on a printing board and the like by etching a conductive film. However, the etching process is not only a cumbersome and costly process but also has problems of a treatment of waste liquid etc. Thus, it is not preferable in an environmental point of view. In addition, there is an additional problem that a usable substrate is limited to one having resistance to etching, such as a plastic film and the like. Furthermore, when conductive paste is used, it is needed that a conductive circuit such as wiring should be formed by a screen printing method. In this case, heating at high temperature is required after the printing in order to obtain good conductivity and the lowest limit of volume resistance thereof is of the order of 10−5 Ω·cm (see Patent Document 1 and 2 below).
On the other hand, it has been recently known that volume resistance of the order of 10−6 Ω·cm can be obtained on a relatively thin film having thickness of 0.1 to 5 μm by use of silver nanoparticles (see Non-Patent Document 1 and 2 below). However, paste containing the silver nanoparticles needs heating and sintering at high temperature of 200° C. or so for several tens of minutes in order to obtain such resistance. As such, this method cannot be applied to an ordinary paper substrate such as a coating paper and a plastic film substrate such as polyester. In addition, this method is disadvantageous in that the adhesiveness between a formed conductive circuit or conductive coating film and a substrate is poor and a crack may be easily formed (see Patent Document 3 and 4 below).
Patent Document 1: JP-A 2000-260224
Patent Document 2: JP-A 2003-16836
Patent Document 3: JP-A 2004-273205
Patent Document 4: JP-A 2005-81501
Non-Patent Document 1: Journal of Japan Institute of Electronics Packaging Society, Vol. 5, No. 6 (2002), pp.
Non-Patent Document 2: Nikkei Nano Business, Vol. 22 (2005, Sep. 26), pp. 2-7